So I have been emailing with Nik from Ceriartone. Very good customer service, I must say. They did build my amp. Here are some excerpts from the emails in case anyone is interested:
I did test an OTS20 today. In clean channel with Vol and MV set to full, there was no squeal. It tends to do it in OD channel, esp if gain set a bit high....the pedal is causing that, most likely. I was talking about with no pedal in. But yeah, if you bring the controls down a bit, it'd be fine I think, with the pedal.
You need to use a multimeter, using millivolts range, in DC. Any simple meter will do. Bias probe might not give correct reading, as the bias socket has 1R to ground, from cathode of tube.